Sunday, October 25, 2015

Reading List: Concrete Planet

Visitors to Rome are often stunned when they see the
Pantheon and
learn it was built almost 19 centuries ago, during the reign
of the emperor Hadrian. From the front, the building has a
classical style echoed in neo-classical government buildings
around the world, but as visitors walk inside, it is the amazing
dome which causes them to gasp. At 43.3 metres in diameter, it was the
largest dome ever built in its time, and no larger dome has, in all the centuries
since, ever been built in the same way. The dome of the Pantheon is a monolithic
structure of concrete, whose beauty and antiquity attests to the
versatility and durability of this building material which has become
a ubiquitous part of the modern world.

To the ancients, who built from mud, stone, and later brick, it must have
seemed like a miracle to discover a material which, mixed with water, could
be moulded into any form and would harden into stone. Nobody knows how or where
it was discovered that by heating natural limestone to a high temperature it
could be transformed into
quicklime (calcium oxide),
a corrosive substance which reacts exothermically with water, solidifying into
a hard substance. The author speculates that the transformation of limestone
into quicklime due to lightning strikes may have been discovered in Turkey and
applied to production of quicklime by a kilning process, but the evidence for this
is sketchy. But from the neolithic period, humans discovered how to make
floors from quicklime and a binder, and this technology remained in use until
the 19th century.

All of these early lime-based mortars could not set underwater and were
vulnerable to attack by caustic chemicals. It was the Romans who discovered
that by mixing volcanic ash
(pozzolan), which was
available to them in abundance from the vicinity of Mt. Vesuvius, it was possible
to create a “hydraulic cement” which could set underwater and
was resistant to attack from the elements. In addition to structures like the
Pantheon, the Colosseum, roads, and viaducts, Roman concrete was used to build
the artificial harbour at
Caesarea in Judea,
the largest application of hydraulic concrete before the 20th century.

Jane Jacobs has
written that the central aspect of a dark age is not that specific things
have been forgotten, but that a society has forgotten what it has
forgotten. It is indicative of the dark age which followed the fall of
the Roman empire that even with the works of the Roman engineers remaining
for all to see, the technology of Roman concrete used to build
them, hardly a secret, was largely forgotten until the 18th century, when a few
buildings were constructed from similar formulations.

It wasn't until the middle of the 19th century that the precursors of modern
cement and concrete construction emerged. The adoption of this technology
might have been much more straightforward had it not been the case that a
central player in it was
William Aspdin, a
world-class scoundrel whose own crookedness repeatedly torpedoed ventures
in which he was involved which, had he simply been honest and straightforward
in his dealings, would have made him a fortune beyond the dreams of avarice.

Even with the rediscovery of waterproof concrete, its adoption was slow in
the 19th century. The building of the
Thames Tunnel by
the great engineers
Marc Brunel and his son
Isambard Kingdom Brunel
was a milestone in the use of concrete, albeit one achieved only after
a long series of setbacks and mishaps over a period of 18 years.

Ever since antiquity, and despite numerous formulations, concrete had one
common structural property: it was very strong in compression (it resisted
forces which tried to crush it), but had relatively little tensile
strength (if you tried to pull it apart, it would easily fracture). This
meant that concrete structures had to be carefully designed so that the
concrete was always kept in compression, which made it difficult to build
cantilevered structures or others requiring tensile strength, such as many
bridge designs employing iron or steel. In the latter half of the 19th century,
a number of engineers and builders around the world realised that by
embedding iron or steel reinforcement within concrete, its
tensile strength could be greatly increased. The advent of
reinforced
concrete allowed structures impossible to build with pure concrete. In
1903, the 16-story
Ingalls Building
in Cincinnati became the first reinforced concrete skyscraper, and the
tallest building today, the
Burj Khalifa in
Dubai, is built from reinforced concrete.

The ability to create structures with the solidity of stone, the strength
of steel, in almost any shape a designer can imagine, and at low cost
inspired many in the 20th century and beyond, with varying degrees of
success. Thomas Edison saw in concrete a way to provide affordable houses
to the masses, complete with concrete furniture. It was one of his less
successful ventures. Frank Lloyd Wright quickly grasped the potential
of reinforced concrete, and used it in many of his iconic buildings. The
Panama Canal made extensive use of reinforced concrete, and the Hoover Dam
demonstrated that there was essentially no limit to the size of a structure
which could be built of it (the concrete of the dam is still curing to
this day). The Sydney Opera House illustrated (albeit after large schedule
slips, cost overruns, and acrimony between the architect and customer) that
just about anything an architect can imagine could be built of reinforced
concrete.

To see the Pantheon or Colosseum is to think “concrete is eternal”
(although the Colosseum is not in its original condition, this is mostly
due to its having been mined for building materials over the
centuries). But those structures were built with unreinforced Roman
concrete. Just how long can we expect our current structures, built from
a different kind of concrete and steel reinforcing bars to last?
Well, that's…interesting. Steel is mostly composed of iron, and iron
is highly reactive in the presence of water and oxygen: it rusts. You'll
observe that water and oxygen are abundant on Earth, so unprotected steel
can be expected to eventually crumble into rust, losing its structural
strength. This is why steel bridges, for example, must be regularly
stripped and repainted to provide a barrier which protects the steel
against the elements. In reinforced concrete, it is the concrete itself
which protects the steel reinforcement, initially by providing an alkali
environment which inhibits rust and then, after the concrete cures, by
physically excluding water and the atmosphere from the reinforcement. But,
as builders say, “If it ain't cracked, it ain't concrete.”
Inevitably, cracks will allow air and water to reach the reinforcement,
which will begin to rust. As it rusts, it loses its structural strength
and, in addition, expands, which further cracks the concrete and allows
more air and moisture to enter. Eventually you'll see the kind of
crumbling used to illustrate deteriorating bridges and other infrastructure.

How long will reinforced concrete last? That depends upon the details. Port
and harbour facilities in contact with salt water have failed in less than
fifty years. Structures in less hostile environments are estimated to have a life
of between 100 and 200 years. Now, this may seem like a long time compared
to the budget cycle of the construction industry, but eternity it ain't, and
when you consider the cost of demolition and replacement of structures such as
dams and skyscrapers, it's something to think about. But obviously, if the
Romans could build concrete structures which have lasted millennia, so can we.
The author discusses alternative formulations of concrete and different kinds
of reinforcing which may dramatically increase the life of reinforced concrete
construction.

This is an interesting and informative book, but I found the author's style
a bit off-putting. In the absence of fact, which is usually the case when
discussing antiquity, the author simply speculates. Speculation is always
clearly identified, but rather than telling a story about a shaman discovering
where lightning struck limestone and spinning it unto a legend about the
discovery of manufacture of quicklime, it might be better to say, “nobody
really knows how it happened”. Eleven pages are spent discussing the
thoroughly discredited theory that the Egyptian pyramids were made of concrete,
coming to the conclusion that the theory is bogus. So why mention it?
There are a number of typographical errors and a few factual errors (no, the
Mesoamericans did not build pyramids “a few of which would equal those
in Egypt”).

Still, if you're interested in the origin of the material which surrounds us
in the modern world, how it was developed by the ancients, largely forgotten,
and then recently rediscovered and used to revolutionise construction, this is
a worthwhile read.